JP3049245B2 - Waveguide type optical modulator - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a waveguide type optical modulator, and more particularly, to a waveguide type optical intensity modulator, a phase modulator, and a polarization scrambler of a high-speed and large-capacity optical fiber communication system. The present invention relates to a waveguide type optical modulator which can be used for a light guide.

[0002]

2. Description of the Related Art With the recent progress in high-speed, large-capacity optical fiber communication systems, niobium acid has been used instead of the conventional direct modulation of a laser diode for reasons such as broadband characteristics, low chirp characteristics, and small propagation loss. The practical use of a waveguide type external modulator using lithium (LiNbO ₃ : hereinafter sometimes abbreviated as LN) is in progress.

[0003] In such a modulator, titanium (T
i) and the like are thermally diffused to form a Mach-Zehnder type optical waveguide, and a traveling wave type signal electrode made of gold (Au) or the like and a ground electrode are provided immediately above or near this optical waveguide. doing. Further, the purpose is to reduce the absorption loss of the light wave guided in the optical waveguide by the traveling wave type signal electrode and the ground electrode, and to reduce the absorption of the light wave and the electric signal propagating through the traveling wave type signal electrode (usually in the microwave band As shown in FIG. 1, a buffer layer 5 made of silicon dioxide (SiO ₂ ) or the like is used to adjust the speed matching with the electric signal (electric signal).
And is provided between them.

[0004]

On the other hand, a waveguide-type optical intensity modulator used in a high-speed, large-capacity optical fiber communication system reduces the load on a high-frequency amplifier and suppresses driving power. Therefore, it is desired to improve the modulation efficiency. For this purpose, it is necessary to shorten the distance between the optical waveguide, the traveling-wave signal electrode, and the ground electrode to lower the driving voltage of the modulator. However, if the buffer layer 5 is provided between the substrate 1 and the traveling-wave signal electrode 3 for the purpose described above, the distance between the optical waveguide 2 and the traveling-wave signal electrode is increased. However, the driving voltage cannot be sufficiently reduced.

When the buffer layer 5 is formed from silicon dioxide or the like, impurities such as iron (Fe) and sodium (Na) may be mixed into the buffer layer 5 during the formation process, or the buffer layer 5 may be deteriorated with time. In some cases, water was absorbed. For this reason, when the buffer layer 5 as shown in FIG. 1 is provided, the modulator characteristics vary depending on the amount of the impurities mixed, and the moisture absorbs the microwave propagating through the traveling wave type signal electrode 3. hand,
There is a problem that the electric signal deteriorates with time.

SUMMARY OF THE INVENTION The present invention provides a new high-speed, high-capacity optical fiber communication system which has a high modulation efficiency by reducing the driving voltage of a modulator and has no change in electrical characteristics with time. It is an object of the present invention to provide a simple optical waveguide type optical modulator.

[0007]

In order to achieve the above object, a waveguide type optical modulator according to the present invention comprises: a substrate having an electro-optic effect; an optical waveguide for guiding an optical wave; A traveling-wave signal electrode and a ground electrode for controlling guided light waves; and a buffer layer provided between the substrate and the traveling-wave electrode. The buffer layer is formed only below the traveling wave signal electrode so as to have a width larger than the width of the traveling wave signal electrode, and at least a part of the buffer layer is formed of the substrate. It is characterized by being buried in the surface layer.

As described above, conventionally, as shown in FIG. 1, between the substrate 1 and the traveling wave signal electrode 3 and the ground electrode 4, the main surface 1a of the substrate 1 on the electrodes 3 and 4 side is formed. The details of how the buffer layer below the signal electrode is involved in the speed matching between the light wave in the optical waveguide and the electric signal propagating in the signal electrode only by forming the buffer layer 5 over the whole. At present, no serious consideration has been made.

In view of the present situation, the present inventors have attempted a detailed study on the buffer layer. As a result, the impedance matching of the electrodes and the speed matching between the light wave and the electric signal are predominantly affected by the buffer layer provided below the traveling wave signal electrode, and provided below the ground electrode. It has been found that the buffer layer hardly affects the characteristics, and that the width of the buffer layer provided below the vicinity of the traveling-wave signal electrode also affects the driving voltage of the modulator. . And, more surprisingly, the driving depends on whether or not a buffer layer provided below the traveling-wave signal electrode is buried in the surface layer portion of the substrate, and on the buried depth of the buffer layer in the surface layer portion. We have also found that voltage is affected.

That is, it has been found that the drive voltage of the modulator can be reduced by forming the buffer layer only below the signal electrode so as to have a width larger than the width of the traveling-wave signal electrode. By embedding at least a part of the substrate in the surface layer of the substrate, it has been found that the driving voltage of the modulator can be further reduced. The present invention
The present invention has been made based on the discovery of the above facts in such a vast research search by the present inventors.

According to the present invention, it is possible to reduce the absorption loss of the light wave guided in the optical waveguide by the electrode and adjust the speed matching between the light wave and the electric signal, as well as to control the driving voltage of the modulator. Can be reduced, and a waveguide type optical modulator with improved modulation efficiency can be obtained.

Further, since the buffer layer is provided only under the traveling wave type signal electrode, the absolute amount of impurities and moisture in the buffer layer can be reduced. Deterioration of an electric signal due to absorption of a propagating microwave can be reduced. The “width of the traveling wave signal electrode” in the present invention refers to the surface width of the traveling wave signal electrode on the side in contact with the buffer layer.

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail with reference to the drawings according to embodiments of the present invention. FIG.
FIG. 1 is a cross-sectional view illustrating an example of a waveguide type optical modulator according to the present invention. The same parts as those of the waveguide type optical modulator shown in FIG. 1 are denoted by the same reference numerals. The waveguide type optical modulator shown in FIG. 2 includes a substrate 1 having an electro-optic effect, an optical waveguide 2 for guiding a light wave, and a traveling wave type for modulating a light wave guided in the optical waveguide 2. It has a signal electrode 6 and a ground electrode 4. The substrate 1 has a width W larger than the width ω only at the lower portion of the traveling wave type optical modulator 3.
And a buffer layer 6 buried in the surface layer portion.

The position of the traveling-wave signal electrode on the buffer layer is particularly determined so that the buffer layer having a width larger than that of the traveling-wave signal electrode is projected on both sides of the traveling-wave signal electrode. It is not limited. However, since the signal electric field applied to each waveguide is made symmetrical and the chirp of the modulator is set to 0, for example, FIG.
As shown in (1), it is preferable to form the traveling-wave signal electrode 3 symmetrically with respect to the center axis 7 of the buffer layer 6.

Further, in the present invention, in order to further reduce the driving voltage of the modulator, the interaction between the electric signal propagating in the traveling wave type signal electrode and the light wave propagating in the optical waveguide is enhanced. Preferably, the width of the traveling wave signal electrode is selected. Specifically, in the case of the waveguide type optical modulator shown in FIG. 2, the ratio W / ω of the width W of the buffer layer 6 to the width ω of the traveling wave signal electrode 3 is set in a range of 1.3 to 6. It is more preferable to set it in the range of 1.5 to 3. The width ω of the traveling wave signal electrode 3 shown in FIG. 2 refers to the surface width of the traveling wave signal electrode 3 on the side in contact with the buffer layer 6 as described above.

The width ω of the traveling wave signal electrode 3 is in the range of 5 to 7 μm according to the width of the waveguide, the set characteristic impedance and the effective refractive index of the microwave. It is preferably set in the range of 0.5 to 42 μm, and more preferably in the range of 7.5 to 21 μm.

The burying depth of the buffer layer in the surface layer of the substrate is not particularly limited as long as the driving voltage of the waveguide type optical modulator can be reduced according to the present invention. However, in the case of the waveguide type optical modulator as shown in FIG. 2, the burying depth d of the buffer layer 6 in the surface layer portion of the substrate 1 is preferably 5 to 10 μm, more preferably 6 to 8 μm. Preferably, there is. Thereby, the driving voltage of the optical modulator can be more effectively reduced, and the effective refractive index of the microwave can be reduced. This reduction in the microwave effective refractive index improves the speed matching with the external modulation signal, and has the additional effect of widening the bandwidth of the waveguide type optical modulator.

FIG. 3 is a sectional view showing a modification of the waveguide type optical modulator shown in FIG. The waveguide type optical modulator shown in FIG. 3 has a buffer layer 8 in which only a portion corresponding to the width p is buried in the surface layer of the substrate 1 at the center of the buffer layer 6 shown in FIG. This is different from the waveguide type optical modulator shown in FIG. As described above, when a part of the buffer layer is buried in the surface layer of the substrate, the effective refractive index of the microwave applied from the traveling wave signal electrode to the light wave guided in the optical waveguide, and the light modulation It is possible to adjust the electrical characteristic impedance of the entire device, and it is possible to make an optimum adjustment according to a desired operating band of the modulator. In addition to this, the excess loss of the light wave guided in the optical waveguide is reduced. Can be suppressed.

FIG. 4 is a sectional view showing a modification of the waveguide type optical modulator shown in FIG. In FIG. 4, the ground electrode 4
Is embedded in the surface layer of the substrate 1. By embedding at least a part of the ground electrode in the surface layer of the substrate in this manner, the driving voltage of the waveguide type optical modulator can be extremely reduced.

The embedding depth of the ground electrode in the surface layer of the substrate is not particularly limited, but in the case of a waveguide type optical modulator having a structure as shown in FIG. 5-
It is preferably 10 μm, more preferably 6 to 8 μm. Further, it is preferable that the burying depth D is the same in the left and right ground electrodes so that the drive voltage of the light wave guided in the left and right optical waveguides 2 divided into two is reduced by the same amount.

FIGS. 5 and 6 are sectional views showing other examples of the waveguide type optical modulator of the present invention. In the waveguide type optical modulator shown in FIG. 5, a passivation film 9 is provided on the main surface 6a of the buffer layer 6 on which the traveling wave type signal electrode 3 is provided.
Is provided. In addition, in the waveguide type optical modulator shown in FIG. 6, in addition to the main surface 6a of the buffer layer 6, the side surface 6b
Is also provided with a passivation film 10. As described above, by providing the passivation film on at least the main surface of the buffer layer on which the traveling-wave signal electrode is provided, the microwave propagation loss caused by moisture penetrating into the buffer layer is further prevented. Unwanted changes with time such as attenuation of electric signals propagating in the traveling wave signal electrode can be further reduced.

Such passivation films 9 and 10
The material that can be used as the material is not particularly limited as long as the material is dense and can prevent moisture from entering the buffer layer 6, but a material such as a dense film can be easily obtained. For this reason, it is preferable that the film be made of at least one of a nitride film such as SiN or Si-ON and a silicon film.

In the present invention, known materials such as silicon dioxide and alumina can be used for the buffer layer. Further, also for the traveling wave signal electrode and the ground electrode, a known metal material having high conductivity such as gold, silver (Ag), and copper (Cu) and easy to be plated can be used.

The substrate in the present invention is not particularly limited as long as it has an electro-optical effect. Lithium niobate, lithium tantalate (Li)
TaO ₃ ) and lead lanthanum zirconate titanate (P
LZT) and the like. Further, when such a material is used, an X-cut plate, a Y-cut plate,
And any substrate such as a Z-cut plate. Further, also in the case where the above-mentioned lithium niobate or the like is used for the optical waveguide, titanium, nickel (Ni), copper, and chromium (Cr) are used from the viewpoint of preventing waveguide loss and deterioration of the electro-optic effect. It is preferable to form the substrate by doping the substrate by a thermal diffusion method.

Hereinafter, a method for manufacturing an optical waveguide type optical modulator according to the present invention will be described with reference to the drawings. At first,
After applying a photoresist pattern for transferring and forming an optical waveguide pattern to a thickness of about 0.5 μm on a substrate 1 made of lithium niobate or the like using a spin coater or the like,
Exposure and development are performed to form an optical waveguide pattern having a development width of 6 to 8 μm. Next, a layer made of an optical waveguide forming material such as titanium is formed to a thickness of about 800 ° on the optical waveguide pattern by a vapor deposition method or the like.
By performing a heat treatment at 0 ° C. for 10 to 20 hours, the substance is diffused into the substrate 1 and the optical waveguide 2 having a width of 8 to 11 μm is formed.
To form

Next, using a chromium mask or the like, dry etching is performed by an electron cyclotron resonance apparatus (ECR) or the like to form a concave portion having a depth corresponding to the depth d of the buried buffer layer in the surface layer of the substrate 1. Form.
Thereafter, a layer made of silicon dioxide or the like is formed on the substrate 1 to a thickness of about 0.5 to 1.5 μm by using a sputtering method so as to fill the recess. Next, in the same manner as described above, dry etching using an electron cyclotron resonance apparatus (ECR) is performed using a chromium mask or the like.
The buffer layer 6 having the width W as described above is formed.
When the ground electrode 4 is buried in the surface layer of the substrate, a recess having a depth corresponding to the depth D of the ground electrode to be buried is formed in the surface layer of the substrate 1 by dry etching as described above. .

Then, a metal such as titanium or nichrome is formed as a base layer on the entire substrate 1 to a thickness of about 0.05 μm by a vapor deposition method or the like, and an electrode material layer such as gold is formed on the entire base layer. 0.2 thickness by evaporation method
It is formed to about μm. Next, on the electrode material layer, after a photoresist is formed to a thickness of about 25 μm by a spin coater or the like, an exposure and development process is performed to form an electrode pattern, and electroplating is performed using the electrode pattern as a mask, The traveling-wave signal electrode 3 and the ground electrode 4 having a thickness of 15 to 20 μm and a width ω are formed.

Next, after removing the remaining photoresist with an organic solvent such as acetone, the underlying layer existing between the traveling wave signal electrode 3 and the ground electrode 4 and under the removed photoresist is removed. And removing the electrode material layer by chemical etching with potassium iodide or the like.

Although not shown in the figure, the final waveguide type optical modulator connects an optical fiber to the entrance and exit of the optical waveguide 2 shown in FIG. It is manufactured by wiring the ground electrode 4 to an electric connector and fixing the substrate 1 on which the optical waveguide 2 and the like are formed to a case made of stainless steel or the like.

[0030]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be specifically described below based on embodiments. Example 1 In this example, an optical waveguide type optical modulator as shown in FIG. 2 was manufactured. An X-cut plate of lithium niobate was used as the substrate 1. On this X-cut plate, a photoresist was formed to a thickness of 0.5 μm using a spin coater, and then exposure and development were performed to obtain a development width of 7 μm. An optical waveguide pattern was formed. Next, a layer made of titanium is formed to a thickness of 800 ° on the optical waveguide pattern by a vapor deposition method,
The titanium was diffused into the substrate 1 by performing a heat treatment at 1000 ° C. for 10 hours in an electric furnace to form an optical waveguide 2 having a width of 9 μm.

Next, while using a chrome mask,
By performing dry etching by ECR, a concave portion having a depth d of 7 μm was formed in the surface layer portion of the substrate 1. Thereafter, a layer made of silicon dioxide was formed to a thickness of 1 μm on the substrate 1 by a sputtering method so as to cover the concave portions. Then, after a chrome mask is set on this layer, E
Dry etching by CR, width W is 13 μm
Was formed.

Next, a layer made of titanium is formed on the entire surface of the substrate 1 to a thickness of 0.05 μm by a vapor deposition method.
A gold layer was formed to a thickness of 0.2 μm on the layer by a sputtering method. Next, after a chromium mask is set on the layer, a photoresist is formed to a thickness of 25 μm by a spin coater, and exposure and development are performed to form an electrode pattern, and electroplating is performed using the electrode pattern as a mask. The thickness of the layer made of gold is
The traveling-wave signal electrode 3 and the ground electrode 4 having a thickness of 5 μm and a width ω of 5 μm were formed.

Next, the remaining photoresist, the titanium underlayer, and the gold deposition layer are removed by the method described in "Embodiment of the invention", and the optical waveguide type optical modulator as shown in FIG. Was prepared. Further, the substrate 1 is fixed to a case (not shown) made of stainless steel.
An optical fiber (not shown) was installed at the entrance and exit of the optical waveguide 2.

When the driving voltage of this optical modulator was examined,
It turned out to be 3.4V. At this time, the impedance of the optical modulator and the effective refractive index of the microwave are 55
Ω and 2.4. Further, when the microwave transmission loss in the microwave propagation band of the electric signal of the optical modulator was measured by leaving it in the air for several days, almost no change with time was observed.

Example 2 In this example, a waveguide type optical modulator as shown in FIG. 4 was manufactured. A waveguide-type optical modulator was manufactured in the same manner as in Example 1, except that a concave portion having a depth D of 7 μm was additionally formed by ECR using a chromium mask. When the drive voltage of this optical modulator was examined, it was found to be 3.0 V. At this time, the impedance of the optical modulator and the effective refractive index of the microwave were 51Ω and 2.4, respectively. Further, when the microwave transmission loss in the microwave propagation band of the electric signal of the optical modulator was measured by leaving it in the air for several days, almost no change with time was observed.

COMPARATIVE EXAMPLE FIG. 1 shows the same manner as in the above embodiment except that the buffer layer 5 was formed over the entire main surface 1a of the substrate 1 without performing dry etching of the buffer layer 6. Such a waveguide type optical modulator was manufactured. As in the example,
When the drive voltage of the optical modulator was examined, it was found to be 4.0 V. At this time, the impedance of the optical modulator and the effective refractive index of the microwave were 54Ω and 2.4, respectively. When the time-dependent change was examined in the same manner as in the example, the microwave propagation loss in the microwave band of the electric signal was such that the 3 dB down value was 10 GHz, but the 3 dB down value moved to 8 GHz. It was found that the optical modulator deteriorated with time.

As is apparent from the above, the waveguide type optical modulator of the present invention has a driving voltage which is substantially equal to the impedance of the optical modulator and the microwave effective refractive index and has no difference in speed matching. It can be seen that it is possible to achieve the reduction of the time-dependent change due to the increase of the microwave propagation loss due to the absorption of moisture in the buffer layer.

As described above, the present invention has been described in detail based on the embodiments of the present invention with reference to specific examples. However, the present invention is not limited to the above-described contents, and can be implemented in any form without departing from the scope of the present invention. Deformation and modification are possible.

[0039]

As described above, according to the waveguide type optical modulator of the present invention, the speed of the electric signal propagating in the traveling wave type signal electrode and the speed of the light wave propagating in the optical waveguide are matched. In addition, the driving voltage of the modulator can be reduced while the microwave propagation loss is kept low, and the modulation efficiency can be improved. Further, since the buffer layer is provided only below the traveling wave signal electrode, variations in modulator characteristics due to impurities in the buffer layer and changes over time of the waveguide type optical modulator due to moisture absorption of the buffer layer are prevented. can do.

[Brief description of the drawings]

FIG. 1 is a sectional view showing an example of a conventional waveguide type optical modulator.

FIG. 2 is a cross-sectional view illustrating an example of a waveguide type optical modulator according to the present invention.

FIG. 3 is a cross-sectional view showing a modification of the waveguide optical modulator shown in FIG.

FIG. 4 is a sectional view showing a modified example of the waveguide type optical modulator shown in FIG.

FIG. 5 is a cross-sectional view showing an example in which a passivation film is provided on a buffer layer in the waveguide optical modulator of the present invention.

FIG. 6 is a cross-sectional view showing another example in which a passivation film is provided on a buffer layer in the waveguide type optical modulator of the present invention.

[Explanation of symbols]

 Reference Signs List 1 substrate 2 optical waveguide 3 traveling-wave signal electrode 4 ground electrode 5, 6, 8 buffer layer 7 center axis of buffer layer 9, 10 passivation film W width of buffer layer ω width of traveling-wave signal electrode d substrate of buffer layer Depth of the part buried in the surface layer part p Width of the part buried in the surface layer part of the substrate of the buffer layer D Depth of the part buried in the surface part of the substrate of the ground electrode

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-8-166565 (JP, A) JP-A-10-3065 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) G02F 1/00-1/035 G02F 1/29-1/313

Claims (5)

(57) [Claims] 1. A substrate having an electro-optic effect, an optical waveguide for guiding a light wave, a traveling-wave signal electrode and a ground electrode for controlling the light wave guided through the optical waveguide,
In a waveguide type optical modulator including a buffer layer provided between the substrate and the traveling wave type signal electrode, the buffer layer is disposed only under the traveling wave type signal electrode and the traveling wave type A waveguide type optical modulator formed so as to have a width wider than a width of a signal electrode, wherein at least a part of the buffer layer is embedded in a surface layer portion of the substrate. 2. The waveguide type optical modulator according to claim 1, wherein at least a part of the ground electrode is buried in a surface layer of the substrate. 3. The waveguide type optical modulator according to claim 1, wherein a width of the traveling wave type signal electrode is smaller than a width of the optical waveguide. 4. The buffer according to claim 1, wherein a passivation film is provided on at least a main surface of the buffer layer on a side where the traveling wave signal electrode is provided. Waveguide type optical modulator. 5. The waveguide type optical modulator according to claim 4, wherein said passivation film comprises at least one of a nitride film and a silicon film.